Beamsplitters are widely used components in many optical systems. For example, one possible use for a beamsplitter is in diffraction-limited optical imaging and measuring systems, such as the thin-film measuring microscope shown in FIG. 1. That microscope is seen to include a broadband light source 11, such as an arc lamp, which provides light that typically has a wavelength range of at least 250-1200 nm and which may extend from 200-2500 nm or beyond. A band-selecting filter 13, such as a reflective filter wheel, may be positioned in the light path to select a particular octave-wide band of interest within this available wavelength range (in order to avoid overlapping diffraction orders in the microscope's spectrometer 27). Typically, a spatial homogenizer 15 is also placed in the light path to eliminate any artifacts of the beam due to the illumination system. Next, the beamsplitter 17 that is of particular interest with regard to the present invention is placed so as to reflect a portion of the light beam through a microscope objective 19 to illuminate an area of the object or workpiece 21 to be viewed and measured. The light reflected or scattered from the workpiece 21 is gathered and focused by the microscope objective 19 onto a reflective field stop 23 having an aperture 25, the light being partially transmitted through the beamsplitter 17 on the way from the objective 19 to the stop 23. In this manner, the illuminated area of the workpiece 21 is imaged on the stop 23. Aperture 25, such as a small slit, may be the entrance to a spectrometer 27, as shown here, having a reflective concave diffraction grating 29 and a detector array 31 and a zero-order detector 36. Light not passing through aperture 25 is reflected by the field stop 23, which typically is oriented at a 45.degree. angle to the optical axis of the system, and is then directed via relay lenses and mirror 33 to viewing optics, such as an eyepiece 35. An aperture stop 34 may be placed in the viewing system between relay lenses 33 to ensure against introducing artifacts into the image enroute to the eyepiece 35. Details of similar microscopes can be found in U.S. Pat. No. 4,844,617 to Kelderman et al., and many other publications.
The beamsplitter 17 in the above described example and beamsplitters in many other optical systems are required to have a large performance bandwidth, i.e. to have a nearly spectrally neutral response over a very broad spectral range. Beamsplitters for use in imaging systems should not introduce any artifacts of their own into the observed image. Further, beamsplitters should be both durable and insensitive to changes in humidity and other environmental factors. Low absorption is critical in those optical systems that need to make efficient use of the available light, such as in spectroscopic systems.
There are essentially two basic types of beamsplitter, namely wavefront-division beamsplitters and area-division beamsplitters, each with its own advantages over the other. Wavefront-division beamsplitters are the type generally preferred for use in imaging systems, because the uniform areawise response of their partially reflective, partially transmissive coatings give them the very fine resolution needed in such systems. However, it is very difficult to make broadband beamsplitters of this type that are low in absorption. Further, beamsplitter coatings made by conventional techniques are not both durable and spectroscopically stable for changes in environmental conditions. In particular, most all-dielectric optical coatings have a demonstrable change in performance as room humidity changes. By contrast, all-fluoride dielectric coatings may be insensitive to humidity, but are instead often somewhat fragile. Coatings that are made by using an ion-assisted deposition technique could be used to make coatings that are both humidity insensitive and durable, but it is unknown whether such coatings would work in both ultraviolet (to at least 254 nm) and visible regimes. Metal-dielectric hybrid coatings achieve broadband performance, but are relatively lossy, and hence less than optimal for use in spectroscopic optical systems and the like. Further, the dielectric materials used in the hybrid coatings are believed to present the same undesirable tradeoff between durability and humidity sensitivity as all-dielectric coatings.
Area-division beamsplitters are also well known For example, U.S. Pat. No. 4,586,786 to Suzuki et al. discloses a beamsplitter having a pattern of randomly arranged light reflecting and light transmitting portions, such as a plurality of aluminum squares or circles arranged on a glass surface. The arrangement is random in order to avoid degradation in the intensity distribution of a point image, due to diffraction if they were to be disposed periodically. However, the arrangement is chosen not to be so random that the irregularity or fluctuation in the quantity of light reaching a sensor surface would exceed 5%. The reflecting portions have a minimum size or width on the order of 1/100 to 1/10 of the distance along the optical axis from the beamsplitter to the image plane, and have a size which is typically equal to that over which a point imaging light beam of F/5.6 illuminates the beamsplitter surface.
The Oriel Corporation of Stratford, Conn., U.S.A. sells a "polka dot" beamsplitter, seen in FIG. 2, having a periodic pattern of 2.5 mm diameter aluminum dots 43, separated by a 3.2 mm center-to-center distance, coated on a flat UV-grade fused silica substrate 41 and covered by a protective silica overcoat. The beamsplitter has very broadband performance, with a response said to be neutral over a 250-2500 nm wavelength range. The reflectance spectrum is characteristic of the thick aluminum film with silica overcoat, while the transmittance spectrum is that of the silica substrate. The smallest beam diameter suitable for a 50% R / 50% T split is 9.5 mm. Oriel Corp. also sells a "coarse grating" beamsplitter consisting of a series of small mirror facets formed on the surface of a glass substrate, with symmetrical triangular profiles spaced at 4/mm. The substrate is coated with aluminum and a protective overcoat. Faceted surface geometries are available for splitting a normal incidence beam into two reflected beams at 45.degree. to the incident beam (at right angles to each other) or at 60.degree. to the incident beam. Again, the reflectance spectrum is that of the overcoated aluminum.
If a "polka dot" beamsplitter were used in the thin-film measuring microscope of FIG. 1, the pupil of the objective 19 would only be partially filled with light in a pattern corresponding to the dots on the beamsplitter. This would necessarily modify the image produced on the reflective stop 23 by the objective 19. The efficiency of the beamsplitter would be hard to predict, because the RT product would depend on how the images of the polka dots, in reflection, lined up with the polka dots themselves. In its sales brochures, Oriel Corp. cautions potential buyers that these beamsplitters should not be used near any focal plane of an imaging system. Likewise, for its "coarse grating" beamsplitters, Oriel Corp. states that since the grating does not function as a continuous surface beamsplitter, it is not recommended for imaging systems.
In U.S. Pat. No. 1,509,936, Douglass describes a light transmitting and reflecting device positioned behind a camera lens for producing duplicate images of an object on two separate sheets of photographic film. Two large-angle prisms are placed together so that their large faces are mutually engaged. The prisms are provided along their large faces with a plurality of parallel, equidistant, triangular cuts or depressions, with the depressions separated from each other by a plurality of parallel light-reflecting surfaces located between the depressions. The prisms, when mutually engaged, form a cube having diagonally therein a series of alternating square gaps and reflecting surfaces, with the gaps corresponding to matching cuts on the large faces providing areas through which light rays may be transmitted. The engaged surfaces between the transmissive gaps require a reflective material coating to effect the light reflection. Accordingly, the reflective spectrum will match that of whatever coating material is used.
It is an object of the invention to provide a broadband area-division beamsplitter suitable for use in diffraction-limited imaging systems.
It is another object of the invention to provide a broadband area-division beamsplitter which is both durable and insensitive to room humidity and other environmental changes, and which has a neutral reflectance response that does not depend upon the reflectance of a coating material.